Layered filtering structure

ABSTRACT

A layered filtering structure ( 10 ) comprises at least a first layer ( 12 ) and a second layer ( 13 ). Each layer comprises a web of metal fibers which has been sintered. The two layers ( 12, 13 ) are in contact with each other. The first layer, most close to the filter inlet side has a porosity below 55%, and the second layer, closer to the filter outlet side has a porosity which is at least 20% greater than the porosity of the first layer. The first layer is compacted in a separate manufacturing step. The layered filtering structure combines the advantage of a small filter rating with a low pressure drop.

FIELD OF THE INVENTION

The present invention relates to a layered filtering structure which isadapted for micro-filtration purposes. The term “micro-filtration”refers to filtering structures which are able to retain particles with amaximum size of between 0.5 μm and 10 μm, in particular below 2 μm.

BACKGROUND OF THE INVENTION

Presently available filter material for applications such asmicro-filtration and for in situ cleanable filtration media convenientlycomprise ceramic membrane layers fixed to the surface of porous sinteredmetal powder or metal fiber substrates. The high pressure drop acrossthese filter laminates, however, is a considerable drawback since thefiltering process requires additional energy due to the high pressureand robust mechanical supports for the filter layers. In addition,repeated backflushing is difficult and, after all, the ceramic layersare quite brittle, which adversely affects durability.

SUMMARY OF THE INVENTION

It is an object of the present invention to avoid the drawbacks of theprior art.

It is also an object of the present invention to provide a filteringstructure adapted for microfiltration without causing high pressuredrops across the structure.

It is still another object of the present invention to provide afiltering structure which allows for repeated backflushing.

According to a first aspect of the present invention, there is provideda layered filtering structure which comprises at least a first layer anda second layer. Each layer comprises a web of metal fibers which havebeen sintered. The two layers are in contact with each other. The firstlayer, at the upstream side, so most close to the filter inlet side, hasa porosity below 55%. The second layer, at the downstream side, socloser to the filter outlet side, has a porosity which is at least 20%greater than the porosity of said first layer. Preferably the porosityof the second layer is greater than 80%.

The first layer with the lower porosity determines the filter rating,i.e. the size of the particles the majority of which still pass thefilter.

The fibers in the first layer preferably have a diameter of less than 3μm, e.g. less than 2.5 μm, e.g. 2 μm.

The first layer preferably has a weight ranging from 300 g/m² to 600g/m² so that its thickness is limited to a range of between 0.05 mm to0.15 mm. Generally, the pressure drop over a filter is aboutproportional to its thickness. In this case, the limited thickness ofthe first layer limits the resultant pressure drop to an acceptabledegree. The degree of pressure drop over the second layer isconsiderably lower than the pressure drop over the first layer due tothe greater porosity of the second layer. So the total pressure dropover the whole filtering structure is about equal to the limitedpressure drop over the first layer. An incoming fluid is immediatelyable to expand in the second layer once it has passed the first layer.

The diameter of the fibers in the second layer is preferably three timesgreater than the diameter of the fibers in the first layer.

In order to obtain the difference in porosity between the first layerand the second layer, the first layer is sintered and compactedseparately and in advance. Only thereafter, this sintered and compactedfirst layer is brought in contact with the second layer and a secondsintering operation takes place to sinter the fibers in the second layerand to bond the two layers together.

The reason for the separate sintering and compacting step of the firstlayer can be explained as follows.

Compacting two or more layers of metal fiber webs together leads to adegree of porosity in the layers which is about the same over the wholelayered structure, and this is to a great extent independent of thefiber diameter in the different layers. This is in contrast with otherfilter media such as filter powders or filter particles. By separatelysintering and compacting the first layer in advance, it is avoided thatthe second layer is compacted to more or less the same degree of thefirst layer.

In a preferable embodiment of the invention, the first layer has atleast one even smooth surface in order to provide a good contact withthe second layer.

A cold isostatic pressing operation is to be preferred for thecompacting operation of the first layer, since this allows obtaining ahomogeneous filter medium. Cold isostatic pressing, however, leads to aslightly rough surface and not to an even and smooth surface. Preferenceis here given to a cold isostatic pressing operation where the pressureis applied from one side and where the other side of the layer rests onan even and smooth support in order to obtain an even and smoothsurface.

Preferably a wire net is fixed as support to the first layer or to thesecond layer. Most preferably the filtering structure is sandwichedbetween a first wire net fixed at the first layer at the inlet side anda second wire net fixed at the second layer at the outlet side. Themeshes of the first wire net are smaller than the meshes of the secondwire net. The diameters of the wires of the first wire net are smallerthan the diameters of the wires of the second wire net.

Next to the function of support the first wire net still has anotherfunction and advantage. This first wire net generates some turbulence inthe incoming flow which improves the anti-fouling behavior.

According to a second aspect of the present invention, there is provideda method of manufacturing a layered filtering structure. The methodcomprises the following steps:

-   -   (a) sintering a web of metal fibers to form a first layer;    -   (b) compacting the first layer to a porosity below 55%    -   (c) providing a web of metal fibers to form a second layer;    -   (d) bringing the first compacted layer and the second layer in        contact with each other to form a layered assembly;    -   (e) sintering the layered assembly to form a coherent entity        wherein the second layer has a porosity which is at least 20%        higher than the porosity of the first layer.

Preferably the compacting step is done by means of a cold isostaticpressing operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described into more detail with reference tothe accompanying drawing wherein

FIG. 1 shows an enlarged view of a cross-section of a layered filteringstructure according to the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

First Example.

Referring to FIG. 1, a layered filtering structure 10 according to theinvention comprises a first layer 12 of a sintered and compacted fiberweb. The fibers in the first layer have a diameter of 2 μm. The weightof the first layer is 450 g/m². The thickness is about 0.10 mm.

The first layer 12 is via a smooth and even surface direct in contactwith a second layer 13 of a sintered fiber web. The fibers in the secondlayer have a diameter of 8 μm. The weight of the second layer is 225g/m².

A first wire net 14 is fixed to the first layer 12. This first wire net14 has been rolled to a thickness of 0.17 mm and has 48 meshes per inch(1 inch=25.4 mm). Its weight is 380 g/m².

A second wire net 15 is fixed to the second layer 13. This second wirenet 15 has a thickness of 0.45 mm and has 40 meshes per inch. Its weightis 1220 g/m².

A filtering structure as shown in FIG. 1 can be made in the followingway.

Steel fibers with a diameter of 2 μm are obtained by means of thetechnique of bundled drawing, such as described in U.S. Pat. No.3,379,000. A first non-woven web is then produced by means of arando-feeder apparatus which transforms the steel fibers into a web suchas disclosed in GB 1 190 844 (which corresponds to U.S. Pat. No.3,469,297). The web is then sintered separately and compacted by meansof a cold isostatic pressing operation carried out at a pressure higherthan 2000 bar to obtain a porosity lower than 55%, e.g. lower than 50%,e.g. 46%. This results in the first layer 12.

Steel fibers with a diameter of 8 μm are obtained with theabove-mentioned technique of bundled drawing and are used to provide asecond non-woven web. This second non-woven web will form the secondlayer 13. The second non-woven web is put on the second wire net 15 andthe first layer 12 is put with its even surface on the second non-wovenweb. A first wire net 14, which has been pre-rolled, is put on the firstlayer. The thus obtained layered assembly is sintered together under alight pressure to obtain the layered filtering structure 10.

The layered filtering structure according to the invention has beensubjected to a conventional textest for measuring the air permeabilityand to a bubble point pressure test. The results are given hereunder.

First layer Whole filter Weight (g/m²) 450 2275 Thickness (mm) 1.02Global porosity (%) 72.22 Textest Average (liter/dm²) 3.00 ± 15% Bubblepoint pressure test Average (Pa) 27000 ± 15%  24520 ± 15%  Filter rating(μm)   1.37 ± 15% 1.51 ± 15%Second Example.

The same tests have also been applied to another layered filteringstructure according to the invention. The difference with the structureof the first example, is now that the first layer is compacted not in anisostatical way. The first layer is now compacted between two platesuntil a thickness of about 0.10 mm is obtained.

First layer Whole filter Weight (g/m²) 450 2275 Thickness (mm) 0.11 1.02Global porosity (%) 46.63 72.22 Textest Average (liter/dm²) 3.72 ± 15%5.72 ± 15% Bubble point pressure test Average (Pa)  18750 ± 15%   17430± 15%  Filter rating (μm) 1.97 ± 15% 2.12 ± 15%

Other embodiments according to the invention may be envisaged. As amatter of example only, following layered filtering structures aregiven:

diameter of filaments in first diameter of filaments in second layerwith low porosity (μm) layer with high porosity (μm) 4 12 6.5 22 8 30

The material used for the filtering structure according to the inventionmay be conventional compositions such as stainless steel 316®,Hastelloy®, Inconel® or Nichrome®. The latter composition can be appliedfor gas filtration at a high temperature.

1. A layered filtering structure having a filter inlet side and a filteroutlet side, said layered filtering structure comprising at least afirst layer on a second layer, each layer comprising a web of metalfibers which has been sintered, said two layers being in contact witheach other, wherein said first layer, most close to the filter inletside has a porosity below 55%, and wherein said second layer, closer tothe filter outlet side has a porosity of at least 80% and which is atleast 20% greater than the porosity of said first layer, wherein saidfirst layer and said second layer include metal fibers having a lengthranging from one-half inch to six inches.
 2. A structure according toclaim 1, wherein said first layer comprises metal fibers with a diameterof less than 3 μm and wherein said second layer comprises metal fiberswith a diameter of at least three times the diameter of the fibers inthe first layer.
 3. A structure according to claim 1 wherein said firstlayer has a weight ranging between 300 g/m² and 600 g/m².
 4. A structureaccording to claim 1, wherein the first layer has at least one evensurface.
 5. A structure according to claim 1, wherein said structurefurther comprises a wire net which is fixed to the first layer or to thesecond layer.
 6. A structure according to claim 1 wherein said structureis sandwiched between a first wire net and a second wire net, said firstwire net comprising wires having a diameter d1, said second wire netcomprising wires having a diameter d2, said first wire net being locatedat the inlet side, said second wire net having meshes and being locatedat the outlet side, and the first wire net having meshes which aresmaller than the meshes of the second wire net and the diameter d1 ofthe wires of the first wire net being thicker than the diameter d2 ofthe wires of the second wire net.
 7. A structure according to claim 6wherein said first wire net is a calandered wire net.
 8. A structureaccording to claim 1 wherein said first layer is obtainable by coldisostatic pressing operation.
 9. A method of manufacturing a layeredfiltering structure, said method comprising: (a) sintering a web ofmetal fibers to form a first layer; (b) compacting said first layer to aporosity below 55%; (c) providing a web of metal fibers to form a secondlayer; (d) bringing said first compacted layer and the second layer incontact with each other to form a layered assembly; (e) sintering saidlayered assembly to form a coherent entity wherein said second layer hasa porosity which is at least 20% higher than the porosity of the firstlayer; wherein said first layer and said second layer include metalfibers having a length ranging from one-half inch to six inches.
 10. Amethod according to claim 9 wherein said compacting is done by a coldisostatic pressing operation.
 11. A method according to claim 9, whereinsaid second layer has a porosity of at least 80%.
 12. A method accordingto claim 9, wherein each layer comprises a non-woven web of metalfibers.
 13. A method of manufacturing a layered filtering structure,said method comprising: (a) sintering a web of metal fibers to form afirst layer; (b) compacting said first layer to a porosity below 55%;(c) providing a web of metal fibers to form a second layer; (d) bringingsaid first compacted layer and the second layer in contact with eachother to form a layered assembly; (e) sintering said layered assembly toform a coherent entity wherein said second layer has a porosity which isat least 20% higher than the porosity of the first layer; whereinbringing the first compacted layer and the second layer in contact witheach other to form a layered assembly occurs after providing a web ofmetal fibers to form the second layer.
 14. A method of manufacturing alayered filtering structure, said method comprising: (a) sintering a webof metal fibers to form a first layer; (b) compacting said first layerto a porosity below 55%; (c) providing a web of metal fibers to form asecond layer; (d) bringing said first compacted layer and the secondlayer in contact with each other to form a layered assembly; (e)sintering said layered assembly to form a coherent entity wherein saidsecond layer has a porosity which is at least 20% higher than theporosity of the first layer; wherein bringing the first compacted layerand the second layer in contact with each other to form a layeredassembly occurs after the second layer is formed.